The invention relates to a system for continuous thermal combustion of matter, such as waste.
This invention relates to a system for continuous thermal combustion of matter such as waste, comprising an incinerator including an inlet for supplying matter to be combusted, an outlet for discharging. combusted matter, a combustion path extending between the inlet and the outlet and along which, in use, by means of conveying means the matter is continuously passed in a conveying direction from the inlet to the outlet for combustion of the matter, and air supply means for supplying optionally heated air to the combustion path, the system further comprising an energy. generator for generating energy in the form of, for instance, steam or electricity on the basis of heat generated in the incinerator, and control means which, depending on the amount of energy generated by the generator, and/or the amount of oxygen in the incinerator, generates at least one control signal for at least setting the amount of matter which is supplied to the incinerator and/or the speed at which the matter is conveyed in the conveying direction through the incinerator, and/or the amount of air which is supplied to the incinerator by means of the air supply means, the control means controlling the at least one control signal such that by the energy generator an amount of energy per unit time is generated which is at least equal to a first value and/or that an amount of oxygen is present in the incinerator which is at most equal to a second value. Such a system is known per se. One of the problems occurring in waste incineration concerns the fluctuations in the process that are caused by the continuously changing waste composition. Strong fluctuations in the waste composition, and hence in the energetic power supplied, can result in strong process fluctuations, such as fluctuations in the temperature of the incinerator. Such process fluctuations can be detrimental to the system. Also, the process fluctuations can entail product fluctuations, such as fluctuations in the amount of generated energy in the form of generated steam and electricity. As a result of this, the yield and quality of these products are lowered. In the known systems, in an attempt to provide a solution to the problems outlined, the control means is employed, which generate the at least one control signal for setting the amount of matter that is supplied to the incinerator and the conveying speed of the matter through the incinerator and/or for setting the amount of air that is supplied to the incinerator by means of the air supply means. The control means control the at least one control signal, such that the fluctuations referred to are damped. One of the problems in turn caused by the control means is that they cannot adequately deal with protractedly low values of the heating value of the waste. If, for instance, the heating value decreases strongly, the amount of energy that is generated will likewise decrease. Also, less oxygen will be consumed, so that the amount of oxygen in the incinerator increases. As a result of these prolonged deviations, the control means, in an attempt to compensate these deviations in energy and/or oxygen, will increase the amount of matter that is supplied to the incinerator, raise the conveying speed of the matter through the incinerator and/or reduce the amount of air that is supplied to the incinerator. However, if the energy production decreases and/or the amount of oxygen in the incinerator increases, for instance due to the matter supplied being wet, the control means lead to still more wet matter being supplied, with the result that the still larger amount of wet matter leads to the combustion of the matter proceeding slower still, so that still less energy is generated and the amount of oxygen in the incinerator rises further. It is also possible that the control means then cause less air to be supplied to the incinerator, since an excess of oxygen is found to be present in the incinerator. The supply of less air, which is generally heated, in turn can also lead to a decrease of the combustion. In response, the control means can result again in more matter being supplied to the incinerator and/or less air being supplied to the incinerator. Eventually, the incinerator may go out as a result of excessive supply of matter to the incinerator and/or insufficient supply of optionally heated air to the incinerator. The object of the invention is to provide a solution to this problem.
To that end, the system according to the invention is characterized in that the system further comprises at least one protective device which lowers the first value or the second value and/or raises the second value when the at least one control signal exceeds a predetermined maximum value.
The protective device according to the invention therefore has as a consequence that the at least one control signal is limited upwards to prevent the full power drive of the at least one control signal. This is achieved by lowering, or raising, the first and/or second value, with the result that the control means are adjusted such that the control means presently strive for a reduced energy production and/or admit an increased amount of oxygen to the incinerator. In practice, this means that when the composition of the matter changes such that the heating value thereof decreases, it is not attempted to compensate a consequent lower energy production by an unlimited increase of the amount of matter that is supplied to the incinerator. In particular, what is also prevented is that, similarly, the amount of air that is supplied to the incinerator is reduced without limitation because an excess of oxygen appears to be present in the incinerator anyway. By virtue of the at least one protective device mentioned, the possibility of the incinerator thus going out is prevented.
Preferably, the protective device comprises an integrator which starts to integrate the difference between the at least one control signal and the predetermined maximum value in time from the moment that the value of the at least one control signal rises so as to become greater than the predetermined maximum value, the protective device being adapted for lowering the first value by a correction value whose magnitude depends on the magnitude of the output signal of the integrator, or for raising the second value by a correction value whose magnitude depends on the magnitude of the output signal of the integrator. In this case, the protective device therefore comprises an integrator which, as soon as the at least one output signal reaches the maximum value, is set into operation, such that the output of the integrator increases in time. The result of this is that the first value (the so-called setpoint imposed on the energy generator) decreases and/or that the second value (the setpoint imposed on the amount of oxygen in the incinerator) increases. The result of this, in turn, is that the magnitude of the at least one output signal of the control means decreases. In particular, further, the protective device is so arranged that when the magnitude of the control signal falls below the predetermined maximum value again while the correction value is not yet equal to zero, the integrating operation of the integrator is continued on the basis of the inverse of the difference between the magnitude of the value of the control signal and the magnitude of the predetermined maximum value until the correction value is equal to zero. This means that the magnitude of the output signal of the control means decreases just so long until it is less again than the maximum value mentioned. If that happens, and when moreover the correction value is not yet equal to zero, the sign of the input of the integrator will change with the result that the correction value will decrease. This in turn has as a consequence that the reduction of the first value will decrease, so that the protection will control the setpoint of the control means back to the original first value. Also the increase of the second value can decrease accordingly to the original second value. If this happens, the original situation has come about again. The protective device will now be set into operation again only when the output signal becomes equal to or greater than the predetermined maximum value.